This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
熿 combines 火 (fire) with 黄 (yellow), but the exact historical development and original meaning are uncertain. It may have originally described a bright or fiery yellow light, but this is speculative.
The kanji visually contains fire 火 and yellow 黄. Imagine a bright yellow flame, but remember that its actual meaning is unclear.
For コウ, imagine a glowing yellow fire that makes a low 'koh' sound as it burns: koh -> コウ.